The DOJ opened a probe into the department in September 2014, one month after police officer Darren Wilson shot and killed Michael Brown, an unarmed black 18 year old. No criminal charges were brought against the officer, who has resigned from the department. A civil-rights investigation into Wilson yielded no charges, the DOJ said Wednesday.

The broader DOJ investigation examined whether the Ferguson police department fostered a culture of bias against African-Americans that could have contributed to the circumstances surrounding Brown’s death.

“Of course, violence is never justified,” Attorney General Eric Holder said at a press conference following the report’s release. “But seen in this context – amid a highly toxic environment, defined by mistrust and resentment, stoked by years of bad feelings, and spurred by illegal and misguided practices – it is not difficult to imagine how a single tragic incident set off the city of Ferguson like a powder keg.”

The investigation also began amid nationwide protests that erupted over a series of police killings of unarmed black males last year, including Eric Garner, who was choked to death by a New York (NYPD) police officer in July; John Crawford III, gunned down by police in an Ohio Wal-Mart in August; Brown, who was killed a few days later; Akai Gurley, shot to death in a housing project stairwell on Nov. 20 by a NYPD officer; and Tamir Rice, a 12-year-old boy shot dead in a park by Cleveland police on Nov. 22. Rice had been holding a toy gun.

Their deaths have led to widespread calls for police reform and brought to the surface longstanding sentiments in the African-American community that they are treated with more suspicion and hostility by police.

Policing the Police

The investigation of the Ferguson police department is one outcome of a federal law, passed in the wake of a notorious incident of police violence: the 1991 case of Rodney King, a black man who was beaten by Los Angeles police after being stopped for speeding. Three years later, Congress passed the Violent Crime Control and Law Enforcement Act, which included a provision that gave the Justice Department unprecedented power to investigate law enforcement agencies for systemic problems — such as use of excessive force, or racial profiling — and force them to implement reforms.

The law is the only tool that exists to compel widespread change within a police department. The Justice Department can threaten to sue a department for constitutional violations, forcing it to enter into a negotiated settlement, such as a consent decree.

“It’s often hard to reform police departments without external intervention,” said Erwin Chemerinsky, dean of the University of California-Irvine law school, and an expert on constitutional policing. “Institutions are resistant to change. None of us like to have somebody outside telling us what to do. And police departments are especially that way. They have their own internal culture.”

In the past 20 years, the Justice Department has launched at least 65 so-called “pattern or practice” investigations of law enforcement agencies, 32 of which have led to agreements to reform, according to an analysis of DOJ data by Stephen Rushin, a professor at the University of Illinois Law School who studies police misconduct.

That’s a small number compared to the nearly 18,000 law enforcement agencies nationwide.

Still, the reforms have had an impact: today, nearly one in five Americans is served by a law enforcement agency that has been subject to a DOJ investigation under this law, according to Rushin’s analysis.

Most investigations zero in on whether, when and how officers are allowed to use force, including deadly force. Also atop the list: a focus on discriminatory policing of minorities — specifically, blacks and Latinos. The DOJ has also examined allegations of gender discrimination, the treatment of people in the LGBT community, and how officers handle people who are mentally ill.

A New Push for Civil-Rights Investigations

The Obama administration has used its power aggressively to take on widespread problems of police brutality, discrimination and other abuse in local jurisdictions, negotiating more settlement agreements than either the Clinton or George W. Bush administrations.

“In case you haven’t heard, the Civil Rights Division is once again open for business,” said Thomas Perez, the assistant attorney general tapped to lead the division, in a 2010 speech. Combating police misconduct, he said, had become an important priority — and pattern or practice investigations were a “critical tool” for bringing change.

Under Attorney General Eric Holder, the Justice Department has opened 20 investigations and negotiated agreements to implement reforms in 15 departments, including major cities like New Orleans, La., Portland, Ore., and Newark, N.J. It currently has nine open investigations.

Even the Justice Department admits flaws in the process. It’s expensive and can take years to fulfill an agreement. In Los Angeles, which is widely considered the most successful test case, it took more than a decade for the police to complete the required reforms, at a cost of $15 million. And the DOJ’s process for choosing departments to investigate, often sparked by a combination of news reports and complaints from local civil-rights groups or public officials, can make the law feel haphazardly applied.

The DOJ assessed its process in 2010, noting that some police chiefs said that federal investigations create a negative stigma that’s difficult to dispel. They urged a more collaborative approach.

But in some departments, it may be the only way to bring about significant change. Charles Ramsey was the chief of the Metropolitan Police Department in Washington, D.C. when he asked the Justice Department to investigate in 1999, following a series of stories in The Washington Post that said that the department killed more residents per capita than police in any other major city. The resulting agreement led to major reforms, significantly reducing the number of police shootings and boosting the department’s credibility in the community, Ramsey said.

“We would not have been able to make the changes we made without the consent decree,” he said at a 2013 conference. “We would have encountered push back from the union, and we would not have obtained the funding.”

What the Justice Department Found in Ferguson

In its Ferguson investigation, the DOJ was looking for a pattern or practice of discriminatory policing, and examining the department’s use of force.

What it found included routine violations of black residents’ civil rights. For example, the DOJ found that black drivers were more than twice as likely as white drivers to be searched during traffic stops, but 26 percent less likely to be found with contraband. Even so, black drivers were more likely to be cited and arrested during a traffic stop.

The report also found that 88 percent of documented police use-of-force cases involved blacks, and in particular juveniles and people with mental health problems or cognitive disabilities.

Some petty offenses appear to be reserved largely for African-Americans. From 2011 to 2013, a full 95 percent of people charged with a crime called “Manner of Walking in Roadway” were black, as were 94 percent of those charged with “Failure to Comply.”

The city used these kinds of citations to generate revenue, the DOJ found. In 2015, the city anticipated raising more than $3 million in fines and fees — more than double the total from five years earlier.

“Many officers appear to see some residents, especially those who live in Ferguson’s predominantly African-American neighborhoods, less as constituents to be protected than as potential offenders and sources of revenue,” the report said.

Next Steps for Ferguson

The Justice Department made 26 recommendations for reform in Ferguson and will insist on a court-enforced agreement to make those changes. The next step for Ferguson will be to decide whether to negotiate or fight federal officials in court. That process can take months, or even years.

Some departments, like Newark, have cooperated with federal authorities. In July 2014, the Justice Department found that Newark police had a pattern or practice of disproportionately stopping and arresting black residents and using excessive force against them. It found that 75 percent of the stops by NPD officers had no justifiable basis, and that some officers stole citizens’ property and money.

Others have fought back. In North Carolina’s Alamance County, Justice Department officials found a pattern or practice of discriminatory treatment by police — in targeting, stops, searches and arrests — of Latino residents. The department refused to cooperate during the investigation, and has refuted the charges.

The DOJ is now suing Alamance County Sheriff Terry Johnson in his official capacity for fostering a culture of discriminatory policing. In the complaint, federal officials said that Johnson told his deputies to “Go out there and catch me some Mexicans,” encouraging them to arrest Latinos, but not others, for minor infractions. Johnson’s attorney has described the charges as baseless.

Ferguson city officials said in a statement on their website that they were reviewing the findings and would hold a press conference later on Wednesday. On the website, the city also outlined steps it has taken to improve relations with the community since Brown’s death. In October, it set up a citizen review board to make recommendations to the mayor. It also began installing body and dashboard cameras for police officers and launched an effort to recruit officers from more diverse backgrounds.

Next Steps for the Nation

In December, President Barack Obama convened a task force on 21st century policing. Its preliminary report, released this week, recommended compiling data on officer-involved shootings and establishing independent investigations of such incidents. It also recommended reducing police use of military equipment during protests, though it stopped short of recommending the widespread use of body cameras for officers, citing privacy concerns.

The task force also recommended that departments work to build trust in communities of color. A survey it conducted found that 72 percent of whites said they were confident officers would treat people of other races the same way; only 46 percent of Hispanics and 36 percent of blacks agreed.

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